The conversion of heat energy or chemical energy to electrical energy, or visa-versa, may be accomplished in a variety of ways. For example, known electrochemical cells or batteries rely on chemical reactions wherein ions and electrons of a reactant being oxidized are transferred to the reactant being reduced via separate paths. Specifically, the electrons are transferred electrically via wiring through an external load where they perform work and the ions are conducted through an electrolyte separator.
However, battery type electrochemical cells can produce only a limited amount of energy because the confines of the battery casing limit the amount of available reactants that may be contained therein. Although such cells can be designed to be recharged by applying a reverse polarity current/voltage across the electrodes, such recharging requires a separate electrical source. Also, during the recharging process, the cell is typically not usable.
Fuel cells have been developed in an effort to overcome problems associated with battery type electrochemical cells. In conventional fuel cells, the chemical reactants are continuously supplied to and removed from the electrochemical cell. In a manner similar to batteries, fuel cells operate by conducting an ionized species through a selective electrolyte which generally blocks passage of electrons and non-ionized species.
The most common type of fuel cell is a hydrogen-oxygen fuel cell which passes hydrogen through one of the electrodes and oxygen through the other electrode. The hydrogen ions are conducted through the electrolyte separator to the oxygen side of the cell under the chemical reaction potential of the hydrogen and oxygen. Porous electrodes on either side of the electrolyte separator are used to couple the electrons involved in the chemical reaction to an external load via an external circuit. The electrons and hydrogen ions reconstitute hydrogen and complete the reaction, while the oxygen on the oxygen side of the cell results in the production of water which is expelled from the system. A continuous electrical current is maintained by a continuous supply of hydrogen and oxygen to the cell.
Mechanical heat engines have also been designed and used to produce electrical power. Such mechanical heat engines operate on thermodynamic cycles wherein shaft work is performed using a piston or turbine to compress a working fluid. The compression process is performed at a low temperature and, after compression, the working fluid is raised to a higher temperature. At the high temperature, the working fluid is allowed to expand against a load, such as a piston or turbine, thereby producing shaft work. A key to the operation of all engines employing a working fluid is that less work is required to compress the working fluid at low temperatures than that produced by expanding it at high temperatures. This is the case for all thermodynamic engines employing a working fluid.
For example, steam engines operate on the Rankine thermodynamic cycle, wherein water is pumped to a high pressure, and then heated to steam and expanded through a piston or turbine to perform work. Internal combustion engines operate on the Otto cycle, wherein low-temperature ambient air is compressed by a piston and then heated to very high temperatures via fuel combustion inside the cylinder. As the cycle continues, the expansion of the heated air against the piston produces more work than that consumed during the lower temperature compression process.
The Stirling engine has been developed to operate on the Stirling cycle in an effort to provide an engine that has high efficiency and offers greater versatility in the selection of the heat source. The ideal Stirling thermodynamic cycle is of equivalent efficiency to the ideal Carnot cycle, which defines the theoretical maximum efficiency of an engine operating on heat input at high temperatures and heat rejection at low temperatures. However, as with all mechanical engines, the Stirling engine suffers from reliability problems and efficiency losses associated with its mechanical moving parts.
In an effort to avoid the problems inherent with mechanical heat engines, Alkali Metal Thermo-Electrochemical Conversion (AMTEC) cells have been designed as a thermo-electrochemical heat engine. AMTEC heat engines utilize pressure to generate a voltage potential and electrical current by forcing an ionizable working fluid, such as sodium, through an electrochemical cell at high temperatures. The electrodes couple the electrical current to an external load. Electrical work is performed as the pressure differential across the electrolyte separator forces molten sodium atoms through the electrolyte. The sodium is ionized upon entering the electrolyte, thereby releasing electrons to the external circuit. On the other side of the electrolyte, the sodium ions recombine with the electrons to reconstitute sodium upon leaving the electrolyte, in much the same way as the process that occurs in battery and fuel cell type electrochemical cells. The reconstituted sodium, which is at a low pressure and a high temperature, leaves the electrochemical cell as an expanded gas. The gas is then cooled and condensed back to a liquid state. The resulting low-temperature liquid is then re-pressurized. Operation of an AMTEC engine approximates the Rankine thermodynamic cycle.
Numerous publications are available on AMTEC technology. See, for example, Conceptual design of AMTEC demonstrative system for 100 t/d garbage disposal power generating facility, Qiuya Ni et al. (Chinese Academy of Sciences, Inst. of Electrical Engineering, Beijing, China). Another representative publication is Intersociety Energy Conversion Engineering Conference and Exhibit (IECEC), 35th, Las Vegas, Nev. (Jul. 24-28, 2000), Collection of Technical Papers. Vol. 2 (A00-37701 10-44). Also see American Institute of Aeronautics and Astronautics, 190, p. 1295-1299. REPORT NUMBER(S)— AIAA Paper 2000-3032.
AMTEC heat engines suffer from reliability issues due to the highly corrosive nature of the alkali metal working fluid. AMTEC engines also have very limited utility. Specifically, AMTEC engines can only be operated at very high temperatures because ionic conductive solid electrolytes achieve practical conductivity levels only at high temperatures. Indeed, even the low-temperature pressurization process must occur at a relatively high temperature, because the alkali metal working fluid must remain above its melt temperature at all times as it moves through the cycle. Mechanical pumps and even magneto-hydrodynamic pumps have been used to pressurize the low-temperature working fluid.
In an effort to overcome the above-described drawbacks of conventional mechanical and thermo-electrochemical heat engines, the Johnson Thermo-Electrochemical Converter (JTEC) system (disclosed in U.S. Pat. No. 7,160,639 filed Apr. 28, 2003) was developed. Referring to FIG. 2, there is shown a typical JTEC system (electrical connections not shown). JTEC is a heat engine that includes a first electrochemical cell 100 operating at a relatively low temperature, a second electrochemical cell 110 operating at a relatively high temperature, a conduit system 112 including a heat exchanger 114 that couples the two cells together, and a supply of ionizable gas (such as hydrogen or oxygen) as a working fluid contained within the conduit system. Each electrochemical cell includes a Membrane Electrode Assembly (MEA).
More particularly, the JTEC heat engine includes a first MEA stack 118 coupled to a high temperature heat source QH (i.e., a high temperature MEA), a second MEA stack 116 coupled to a low temperature heat sink QL (i.e., a low temperature MEA), and a recuperative heat exchanger 114 connecting the two MEA stacks 116, 118. Each MEA stack 116, 118 includes a non-porous membrane 120 capable of conducting ions of the working fluid and porous electrodes 122 positioned on opposite sides of the non-porous membrane 120 that are capable of conducting electrons.
MEAs have been used in the fuel cell community to generate power via electrochemical reactions involving a fuel and an oxidizer, such as hydrogen and oxygen. However, the MEA stacks in conventional fuel cell applications require bidirectional flow in at least one of the electrodes. For example, oxygen flow into the cathode side of hydrogen-oxygen fuel cells must be maintained as the same time that the hydrogen-oxygen reaction product, water, is exiting. As such, large flow cross-sections for fuel and the oxidizer/reaction product must be an inherent feature of the design of conventional MEA stacks for fuel cells.
No such bidirectional flow is required in the JTEC. Specifically, during operation of the JTEC, the working fluid passes through each MEA stack 116, 118 by releasing an electron to the electrode 122 on the entering side, such that the ion can be conducted through the membrane 120 to the opposite electrode 122. The working fluid is reconstituted within the opposite electrode 122 as it re-supplies electrons to working fluid ions as they exit the membrane 120. The low temperature MEA stack 116 operates at a lower voltage than the high temperature MEA stack 118. The low temperature MEA stack 116 compresses the working fluid at low voltage and the high temperature MEA stack 118 expands hydrogen at high voltage. The difference in voltage between the two MEA stacks 116, 118 is applied across the external load. The hydrogen circulates continuously inside the JTEC heat engine and is never consumed. The current flow through the two MEA stacks 116, 118 and the external load is the same.
Specifically, in the JTEC heat engine, a hydrogen pressure differential is applied across each MEA stack 116, 118 with a load attached, thereby producing a voltage and current as hydrogen passes from high pressure to low pressure. The electron current is directed to the external load as electrons are stripped from the protons as they pass through the membrane 120, which is a proton conductive membrane (PCM). The JTEC system utilizes the electrochemical potential of hydrogen pressure applied across the PCM 120. More particularly, on the high pressure side of MEA stack 116 and the low pressure side of MEA stack 118, hydrogen gas is oxidized resulting in the creation of protons and electrons. The pressure differential at the high temperature end forces the protons through the membrane 120 causing the electrodes 122 to conduct electrons through an external load, while the imposition of an external voltage forces protons through the membrane at the low temperature end. On the high pressure side of MEA stack 116 and the low pressure side of MEA stack 118, the protons are reduced with the electrons to reform hydrogen gas.
Unlike conventional fuel cells, in which the hydrogen exiting the MEA stack would encounter oxygen and react with it producing water, there is no oxygen or water in the JTEC system. This process can also operate in reverse. Specifically, if current is passed through the MEA stack 116, a low-pressure gas can be “pumped” to a higher pressure. The reverse process is rather similar to that of using a MEA stack to electrolyze water, wherein water molecules are split and protons are conducted through the PCM, leaving oxygen behind on the water side. Hydrogen is often supplied at a high pressure to a pure hydrogen reservoir via this process.
In the JTEC, using hydrogen as the ionizable gas (i.e., the working fluid), the electrical potential due to a hydrogen pressure differential across the PCM 120 is proportional to the natural logarithm of the pressure ratio, and can be calculated using the Nernst equation:
                                          V            OC                    =                                    RT                              2                ⁢                F                                      ⁢                          ln              ⁡                              (                                                      P                    H                                                        P                    L                                                  )                                                    ,                            Equation        ⁢                                  ⁢        1            
where VOC is open circuit voltage, R is the universal gas constant, T is the cell temperature, F is Faraday's constant, PH is the pressure on the high pressure side, PL is the pressure on the low pressure side, and PH/PL is the pressure ratio. E.g., Fuel Cell Handbook, J. H. Hirschenhofer et al., 4th Edition, p. 2-5 (1999).
The voltage generated by the MEA stack 116 is thus given by the Nernst equation. The voltage is linear with respect to temperature and is a logarithmic function of the pressure ratio. FIG. 1 is a plot of the Nernst equation for hydrogen and shows the voltage vs. temperature relationship for several pressure ratios. For example, referring to FIG. 1, at a pressure ratio of 10,000, when the temperature is relatively high, the voltage is similarly relatively high and when the temperature is relatively low, the voltage is similarly relatively low.
The working fluid in the JTEC is compressed in the low temperature electrochemical cell 100 by supplying current at a voltage that is sufficient to overcome the Nernst potential of the low temperature cell 100, thereby driving hydrogen from the low pressure side of the membrane 120 to the high pressure side. On the other hand, the working fluid is expanded in the high temperature electrochemical cell 110 as current (power) is extracted under the Nernst potential of the high temperature cell 110. Electrical current flow is generated as hydrogen expands from the high pressure side of the membrane 120 to the low pressure side. As in any thermodynamic engine employing a working fluid and consistent with the nature of compressible gas, in the JTEC, a greater amount of work (electricity) is extracted during high temperature expansion than the work (electricity) input required for the low temperature compression. The difference in heat energy input to the engine to maintain constant temperature during high temperature expansion versus the heat energy removed to maintain constant temperature during low temperature compression is provided as the difference in electrical energy output by the high temperature expansion process versus that consumed by the low temperature compression process.
Consistent with the Nernst equation, the high temperature cell 110 will have a higher voltage than the low temperature cell. Since the current (I) is the same through both cells 100, 110, the voltage differential means that the power generated through the expansion of hydrogen in the high temperature cell 110 is higher than that of the low temperature cell 100. The power output by the high temperature cell (VHT*I) is sufficient to drive the compression process in the low temperature cell 100 (VLT*I) as well as supply net power output to an external load ((VHT*I)−(VLT*I)). This voltage differential provides the basis for the JTEC engine.
Operation of the JTEC is generally similar to any other engine. For example, in a typical jet engine, the compressor stage pulls in air, compresses the air, and supplies the compressed air to the combustion chamber. The air is then heated in the combustion chamber and expands through the power stage. The power stage couples shaft work back to the compressor stage, in order to maintain a continuous supply of compressed air. The difference in work generated by the power stage and that consumed by the compressor stage is the net work output by the engine. However, the primary difference between such conventional engines and the JTEC is that such conventional engines utilize a turbine (i.e., a mechanical device) and operate on the Brayton thermodynamic cycle, whereas the JTEC is an all solid-state engine that operates on the more efficient Ericsson cycle, which is equivalent to the Carnot cycle.
Referring to FIG. 3, there is shown the ideal temperature entropy diagram for the Ericsson engine cycle of the JTEC. Reference numerals “1” through “4” in FIGS. 2-3 represent different thermodynamic states. The thermodynamic states 1 through 4 are identical at the respective identified points in FIGS. 2 and 3. As shown in FIG. 2, beginning at the low-temperature, low-pressure state 1, electrical energy Win is supplied to the low-temperature MEA stack in order to pump hydrogen from the low-temperature, low-pressure state 1 to the low-temperature, high-pressure state 2. The temperature of the hydrogen is maintained nearly constant by removing heat QL from the PCM 120 during the compression process. The membrane 120 is relatively thin (i.e., less than 10 μm thick), and thus will not support a significant temperature gradient, so the near isothermal assumption for the process is valid, provided adequate heat is transferred from the membrane 120 through its substrate.
From state 2, the hydrogen passes through the recuperative, counterflow heat exchanger 114 and is heated under approximately constant pressure to the high-temperature state 3. The heat needed to elevate the temperature of the hydrogen from state 2 to 3 is transferred from hydrogen flowing in the opposite direction through the heat exchanger 114. At the high-temperature, high-pressure state 3, electrical power is generated as hydrogen expands across the MEA stack 118 from the high-pressure, high-temperature state 3 to the low-pressure, high-temperature state 4. Heat QH is supplied to the thin film membrane 120 to maintain a near constant temperature as the hydrogen expands from high-pressure state 3 to low-pressure state 4. From state 4 to state 1, the hydrogen flows through the recuperative heat exchanger 114, wherein its temperature is lowered by heat transfer to hydrogen passing from state 2 to 3. The hydrogen is pumped by the low-temperature MEA stack 100 from state 1 back to high-pressure state 2 as the cycle continues.
However, some challenges have been encountered with developing a JTEC that is suitable for widespread use, particularly for systems that use hydrogen as the working fluid. For example, hydrogen leakage through small defects in the conduit system may occur due to the small size of the hydrogen molecule. In particular, hydrogen leakage can occur at the joints of the interconnects for the conduit couplings between the high-temperature cell and the low-temperature cell.
The engine design is also complicated by the need for a large membrane/electrode surface area and by the need for a significant number of cells to be electrically connected in series to achieve practical output voltage levels. Specifically, unlike conventional fuel cells, where the open circuit voltage can be greater than 1V, the Nernst voltage from the hydrogen pressure differential across a MEA stack is in the range of only about 0.2 Volts. As such, many cells will have to be connected in series to achieve useful output voltage levels.
Further, in order to achieve efficient energy conversion, the membranes must have high diffusion barrier properties, because diffusion of working fluid (such as hydrogen gas) under the pressure differential across the membrane results in reduced electrical output and efficiency. The membranes utilized must also have good ion conductivity. However, known and available membrane materials that have good ion conductivity, such as Nafion manufactured by the DuPont Corp., generally have very poor molecular diffusion barrier properties. Conversely, known and available membrane materials that have high molecular diffusion barrier properties generally have relatively low ionic conductivity, and use of such materials would result is high system impedance and high polarization losses. As such, large membrane areas are needed in order to keep current density at a minimum so as to minimize resistive polarization losses. However, the cell will have low internal impedance if the ion conduction cross-sectional area of the membrane is too large.
Accordingly, there is a need for a practical way of using available high barrier, low conductivity membrane materials to provide a thermo-electrochemical heat engine that can approximate a Carnot equivalent cycle, that can operate over a wide range of heat source temperatures, and that eliminates the reliability and inefficiency problems associated with mechanical engines. The solid state heat engine of the present invention fulfills this need.